The purpose of the proposed project is to study basic mechanisms involved in the repair of DNA, with particular emphasis on post-replication, or recombination-dependent repair. For this project we plan to use two bacterial species, Escherichica coli and Bacillus subtilis. The former has a well-characterized genetic system, and is amenable to analysis by conjugation and transduction. In addition, a wide variety of mutants has been isolated, sensitive to radiation and to mutagenic chemicals. In many cases the specific enzymatic defect has been identified. A large number of mutants has also been isolated from B. subtilis. This organism, because it is amenable to genetic transformation by purified DNA, provides an assay for the biological activity of biochemical intermediates in repair. We plan to examine diverse mutants for their repair capacities in an attempt to broaden our understanding of the genetic control of post-replication repair.